LyX

In my first years at the university, I always got frustrated by having to manually reedit almost every single paragraph of my texts in order to get a nearly decent look out of it and to avoid orphans and widows. And if you edit in one place for layout purposes only, you automatically mess up somewhere else. Also, there were still errors slipping in: one space to many, a font not adjusted or with wrong size...
I had heard of (La)Tex back then, but I considered it too hard to learn. Then one day I stumbled over LyX, I don't even exactly remember how and where. I'd have to lie if I claimed all became better on the instant, in fact I was heavily confused in the first hour of trials. You have to dump almost all of your concepts of how to use text software, but if you do, it is more than worth it!
Unlike other text editors, you don't see this typewriter style user interface, you get no empty sheet which you fill line by line and do the layout as you type. What you get is a plain field where you type your text, you don't care about line breaks or font size here. You mark words or paragraphs as headings or quotes and of course you can use italics and other methods of highlighting.
The layout happens when you click "Generate PDF" and *shazaam* you are seeing a pdf file with your text, properly set and with an automatically generated Table of Contents according to your headings. (No more counting of page numbers, no more forgetting to change the ToC!). You don't rely on your spacebar or linebreaks for editing purposes, it is all done by LyX in a sane manor.
The included help files are well written and huge, also these are LyX documents themselves, so message and medium are identical and you gain even more from reading both their source and the generated pdf! Learning curve is steep as a rocket's course and it is not even difficult, you don't have to learn cryptic commands by heart or search submenu jungles. All things needed in daily live are just one click away most of the time. (You can, if you like, reach every single function by keyboard shortcuts)
I need to write quite long texts with lots of quotations and some images at times, so I value that the bibliography is generated from bibtex files and the images get their own ToC where I need it, for example in the Appendix, instead of the front ToC. Math and formulas in general are done very well, too.
Minor problems: the frontpage layout gave me headaches at times, when I wanted to use lots of the features the underlying LaTex has avaiable. That and some small glitches are the only let downs I found so far, but given the complexity and the abilities this software has, they are not critical.
All in all, this tool gave me the calmness to focus on the content of my texts completely and saved me from pains I had with the common word processors of office suits.